1. Field of Invention
The present invention relates generally to light emitting devices and more particularly to light emitting diodes using phosphors.
2. Description of Related Art
Blue light emitting diodes (LEDs) are used with luminescent materials (phosphors) to produce light emitting devices which emit apparently white light. U.S. Pat. Nos. 5,813,753 and 5,998,925, for example, disclose light emitting devices in which a blue LED is disposed in a reflective cup and surrounded by material including phosphors. A portion of the blue light emitted by the LED is absorbed by the phosphors, which in response emit red and green light. The combination of the unabsorbed blue light emitted by the LED and the red and the green light emitted by the phosphors can appear white to the human eye. Unfortunately, such conventionally phosphor surrounded LEDs frequently emit white light in a central cone surrounded by annular rings of yellow and blue light, rather than emitting light in a uniformly white spatial profile.
Lowery U.S. Pat. No. 5,959,316, incorporated herein by reference, discloses that such annular rings of blue and yellow light can result from nonuniformity in the thickness of the phosphor containing material surrounding the LED and consequent spatially nonuniform absorption of blue light and emission of red and green light. In particular, thick regions of phosphor containing material absorb more blue light and emit more red and green light than do thin regions of phosphor containing material. The thick regions thus tend to appear yellow, and the thin regions tend to appear blue. Lowery also discloses that deposition of a transparent spacer over and around the LED prior to deposition of a uniform thickness of phosphor containing material can eliminate the annular rings in the emission profile. However, surface tension makes the shape and thickness of the phosphor containing material, typically deposited as a liquid or paste (solids dispersed in a liquid), difficult to control.
What is needed is a method for uniformly coating an LED with phosphor containing material.